There are a number of different positioning systems in existence today based on various principles including ultrasound and radio waves. These systems all have both advantages and disadvantages. The system according to the invention employs identification tags which are placed on the objects to be monitored. The identification tags contain an ultrasonic transmitter, radio transmitter and radio receiver.
Systems based on radio waves, for example in one of the unlicensed ISM bands (Industrial, Scientific, Medical), for example 2.4 GHz for WLAN or Bluetooth™ plug-in wireless network card in PCs have the following advantages: they have a high data transfer rate, they can tolerate a lot of movement (Doppler shift) and have a long range. The disadvantages of such a system are that the long range makes it necessary to use 3 or more base stations in order to obtain a position.
Location systems based on ultrasound have the following advantages: they have a short range, the waves do not pass through walls, i.e. they are easy to position at room level and the detectors are cheap. The disadvantages are that these systems have a low transfer capacity and cannot tolerate much movement.
The invention outlined herein combines the best features of radio and ultrasound-based systems. The system may advantageously be employed in areas which are traditionally used for only radio-based systems or only ultrasound-based systems.
By using ultrasound for calculating transit time differences of ultrasonic pulses at different locations in a room or area at the same time as radio waves for transferring the ID code and other information, the system attains a high data transfer capacity and becomes insensitive to Doppler shift.
Prior Art
Several different principles exist today for locating objects within a limited area. The systems are generally based on ultrasonic and/or radio communication.
U.S. Pat. No. 6,433,689, which is the applicant's own patent, is an example of a system based on ultrasound. In this patent a system is described for control and surveillance of objects or people. This is implemented by attaching identification tags to the object that is required to be monitored. The tags, which have a unique identification code, are provided with a transmitter and receiver for communicating by ultrasound as well as sound in the audible range. The present invention differs from this in that it employs both ultrasound and radio waves.
U.S. Pat. No. 6,141,293 describes a system where ultrasound is combined with radio waves. The difference from the present invention is that the fixed units (the detectors) initiate a positioning by transmitting a radio signal which is received by identification tags transmitting ultrasound.
This also applies to U.S. Pat. No. 6,317,386 which describes a system combining ultrasonic and radio waves. The system works in such a manner that the identification tags are called up by means of radio waves, while the actual communication between transmitter units and base stations is conducted by means of ultrasound. The object of this system, which is for indoor use, is to increase the capacity of an ultrasound-based system. This is accomplished by periodically calling up each tag, which has a unique address, by means of radio waves. The identification of each tag therefore does not need to be transmitted to the base station by means of ultrasound. The area of application of this system is restricted, the system requires accurate location of all the receivers in advance and complicated signal processing, and it functions best when there is an unobstructed view between the transmitter and the receivers. The present invention is also a system which combines ultrasonic and radio waves. However, unlike U.S. Pat. No. 6,141,293 and U.S. Pat. No. 6,317,386 it is the tag itself which initiates transmission of ultrasound, and all information concerning transit time differences of ultrasonic pulses and identification of the tag transmitting the ultrasonic pulses is transmitted from a master unit to a central processing unit. By using such a system a wider area of application is achieved than with the system described in U.S. Pat. No. 6,141,293 and U.S. Pat. No. 6,317,386. The system, moreover, will not be sensitive to movement of the identification tag when it is transmitting signals, and the system does not need extensive calibration.
U.S. Pat. No. 6,121,926 also describes a system for location of identification tags. In this case the tags are attached to objects in a logistics system. When an identification tag transmits a signal with its identification, it has to be received on 3 or more base stations, where an analysis is made of transit time differences and the position is determined. In practice, the accuracy of such a system will be a few meters, thus making it difficult to determine which room an object is located in without extensive calibration. Moreover, the cost per base station with antenna for the radio is substantially greater than the cost of the ultrasonic transmitters, even though one ultrasonic transducer often has to be placed in each room. U.S. Pat. No. 6,121,926 essentially describes an improved signal processing method for improving the location of identification tags by distinguishing between direct waves and reflected waves. This requires processing power which is also not required in the present invention.